Various types of high-pressure discharge lamps use ceramic discharge vessels. These discharge vessels can be used with sodium high-pressure lamps, as well as with metal-halide discharge lamps. The color rendition and color indices of the emitted light can be improved when using a ceramic discharge vessel over that of a glass vessel, since the ceramic material permits a higher operating temperature with respect to a vessel made of glass, typically of quartz glass. Lamps of this type may have power ratings for example of between 100 to 250 W.
Problems arise in the lead-through arrangements for electrical energy to the electrodes in the discharge vessel. Various through-melting technologies are known, primarily from the technology developed for sodium high-pressure discharge lamps. Tubular lead-throughs made of niobium or tantalum are frequently used. These lead-throughs are then melt-sealed by a sealing glass in a ceramic end plug which is fitted into openings formed in the end of the discharge vessel. An arrangement of this type is described, for example, in British Patent 1,465,212, Rigden.
Niobium as well as tantalum have thermal coefficients of expansion which are similar to that of the ceramic used. The thermal coefficient of expansion is about 8.times.10.sup.-6 /.degree.K.
The known sealing arrangements cannot be used, unfortunately, for metal-halide lamps intended to have a long lifetime, and designed for good color rendition, since the
metal-halide fill has the tendency to corrode the niobium lead-through as well as the sealing glass used to seal the niobium tube through the ceramic plug. Sodium high-pressure lamps, likewise, after extended use attack the lead-through, and hence the lifetime of the lamp is limited by attack of condensed sodium on the lead-through arrangement.
Various types of sealing glasses have been used, for example calcium aluminate glass (see the aforementioned British Patent 1,465,212, Rigden) as well as special sealing glasses made of materials which are known by themselves, such as mixtures of aluminum and earth alkali oxides. The referenced U.S. Pat. Nos. 4,501,799, Driessen et al, and 4,740,403, Oomen et al, describe materials which are particularly resistant to attack by halides.